Fuel cells including polymer electrolyte fuel cells (PEFCs) have been expected to serve as next-generation power-generation systems. Among others, since PEFCs have advantages of lower operation temperature and compactness compared with other fuel cells, PEFCs have been expected to be used as home and automobile power supplies.
Under these circumstances, with the recent popularization of fuel cells, not only simply superiority of catalysts for polymer electrolyte fuel cells in terms of activity, but also various improvements, in particular, reductions in amounts of platinum group metals used for the catalysts or supported to carriers have increasingly been demanded, and many studies have been conducted to solve the problems.
Among others, as one of conventional arts that has recently attracted a great deal of attention, a technology for reducing the amount of platinum using a catalyst that has a core-shell structure has been known.
Now, the core-shell structure will be described with reference to the cross-section diagrams in FIGS. 1A and 1B. A structure in which a shell metal 2 is formed by use of a precious metal (e.g. platinum) that delivers catalysis performance for fuel cells, on the surface of a core metal 1 that is formed of an inexpensive metal, is called a core-shell structure. Specifically, the core-shell structure refers to a structure in which the shell metal 2 is formed over the entire surface of the core metal 1 as shown in FIG. 1A, or on a part of the surface of the core metal 1 as shown in FIG. 1B.
Details on the conventional arts will be described. For example, a method for synthesizing a catalyst with a core-shell structure using gold that serves as the core metal 1, and platinum that serves as the shell metal 2 can be mentioned. A reducing agent is added to a solution in which a precursor of gold has been dissolved, to synthesize gold nanoparticles. Then, gold nanoparticles are purified through steps such as centrifugal separation and washing. Subsequently, the gold nanoparticles are added to a solution in which a precursor of platinum has been dissolved, to synthesize metal particles that each have an Au—Pt core-shell structure, and the metal particles each having the core-shell structure are purified through steps such as centrifugal separation and washing. By further dispersing the purified metal particles that each have the core-shell structure, and a carbon material in a solution, a catalyst including the metal particles that each have the core-shell structure and that are supported on the carbon material is synthesized (for example, see JP-A-2010-92725).
Furthermore, as another technique, the following method can be mentioned. For example, a palladium salt, and a carbon powder are mixed/stirred, and then, the mixture is subjected to reduction/filtration/washing, thereby obtaining a palladium-supported carbon powder that carries palladium particles. Then, a predetermined voltage is applied to the palladium-supported carbon powder in a copper solution in which a precursor of copper has been dissolved, thereby depositing Cu on the surfaces of palladium particles. Then, the resulting product is subjected to filtration/washing, thereby obtaining a Pd/Cu core-shell carbon powder that carries particles each having a Pd/Cu core-shell structure. Then, the Pd/Cu core-shell carbon powder is soaked in a solution in which a precursor of platinum has been dissolved. This results in dissolution of Cu and deposition of PT due to a relation of an ionization tendency between Pt and Cu. Accordingly, a Pd/Pt core-shell catalyst that carries, on the carbon powder, particles having a Pd/Pt core-shell structure is obtained (for example, see JP-A-2012-157833).